It is known that the solution polymerization process for producing a styrene-butadiene copolymer rubber by using a catalyst consisting of an organic lithium compound under usual conditions suffers from the disadvantage that the resultant copolymer molecule contains chain blocks consisting of a plurality of styrene units sequentially combined to each other. These styrene units are so-called block styrene.
In order to eliminate the above-mentioned disadvantage, various attempts were made to prepare a random styrene-butadiene copolymer.
In one attempt disclosed in U.S. Pat. No. 2,975,160, the copolymerization of styrene and butadiene was carried out in the presence of a polar organic solvent. In another attempt disclosed in U.S. Pat. No. 3,294,768, the copolymerization of styrene and butadiene was carried out in the presence of an organic salt or complex of sodium, potassium, or the like. In still another attempt, during the copolymerization procedure, one of the monomers to be copolymerized was added intermittently to the copolymerization mixture. In still another attempt, a copolymerization mixture was allowed to separate into a polymer phase and an inert diluent phase and the copolymerization was carried out under the separated phase condition.
The above-mentioned attempts resulted in decreasing the content of the block styrene units in the resultant copolymers. However, the resultant copolymers contained an increased amount of butadiene units having a 1,2-vinyl structure. Also, the modes of bonding of the styrene units and sequence distribution of the styrene units in the resultant copolymers, have not yet been clarified. That is, the copolymers obtained by the above-mentioned attempts should be distinguished from the random styrene-butadiene copolymer rubber of the present invention.
Recently, it has become possible to determine the sequence distribution of the styrene units in styrene-butadiene copolymer, by applying NMR spectrum analysis (Rubber Chemistry and Technology, 43, 1138 (1970))to the copolymer. In this analysis, concrete concepts concerning a long chain block styrene segment and short chain block styrene segment were introduced into the chemistry of the styrene-butadiene copolymer. Also, it was attempted to provide a pseudorandom styrene-butadiene copolymer rubber and a method for producing the same. For example, British Pat. No. 1,446,183 and Japanese patent application Laid-Open No. 53-69288 (1978) disclose the above-mentioned attempt. However, it was unsatisfactory to quantitatively determine the amounts of the long and short chain block styrene segments by means of the NMR spectrum analysis using an analog computer. Also, it was impossible to determine the amount of isolated styrene units in the copolymer by means of the NMR spectrum analysis. Therefore, it was impossible to imagine the concept of random styrene-butadiene copolymer rubber.
Also, the conventional styrene-butadiene copolymer exhibited unsatisfactory resilience, tensile strength, resistance to abrasion, and heat buildup property.